1. Field of the Invention
The present invention relates to an optical modulation system, and more particularly, to the interconnection of external electrodes to an optical modulator so as to minimize loss of signal energy and to prevent the introduction of spurious modes into the signal within the optical modulator.
2. Discussion of the Related Art
In a general fiber optical communication system, optical signals are sent along an optical fiber communication line to a desired location. Optical modulators with performance in the 40 GHz frequency range and beyond, are critical components in optical communication systems.
To achieve high-frequency operation in LiNbO3, the electrical and optical velocity of the modulating and modulated signal must be matched. This is achieved by employing thick ( greater than 10 xcexcm) electrodes in conjunction with a buffer layer (typically SiO2). The buffer layer is deposited directly on the LiNbO3, and the electrode structure is delineated on the buffer layer. While the buffer layer facilitates velocity matching, it also results in decreased modulation efficiency because the applied voltage is partially dropped across the buffer layer. LiNbO3 is an anisotropic material, with the following dielectric constants:
∈extra-ordinary≈26, ∈ordinary≈43 
Thus, planar and uni-planar transmission lines such as microstrip, coplanar waveguide (CPW) and coplanar strip (CPS) tend to be very dispersive when built directly on LiNbO3. As the frequency increases, the fields become more concentrated in the regions below the metal strips, where the substrate permittivity has already resulted in a relatively larger electric displacement since the fields are forced into the LiNbO3 to an increasing extent as the frequency increases. Therefore, a frequency dependent effective permittivity can be defined for the transmission line.
FIG. 1 illustrates a conventional optical modulator device. The modulator has a mounting base 1 that is typically conductive or a non-conductive material covered with a conductive layer. The mounting base 1 is typically at the ground potential of the device and will herein be referred to as the grounded base 1. The optical modulator has an optical modulator chip 2, for example a LiNbO3 chip covered with an insulating buffer layer, mounted on the grounded base 1. The grounded base 1 includes input/output optical terminals 6 and input/output electrical terminals 7. The optical modulator chip 2 has two ground electrodes 3/3xe2x80x2 and a signal electrode 4 mounted on top of the buffer layer above the waveguide 5 that runs across the center of the optical modulator chip 2. This electrode configuration is known as a coplanar-waveguide (CPW). When the electrode structure of the optical modulator chip 2 comprises just one signal electrode, and one ground plane, it is known as a coplanar-strips (CPS) configuration.
Once the electric fields of the signal electrode penetrate through the buffer layer into the optical modulator chip several other effects could occur. Depending on frequency, a CPW mode may couple with other extraneous electrical modes that the structure of the optical modulator device can support. These modes could either be highly dispersive slab modes, or could be zero-cut-off modes. Examples of extraneous modes are: transverse-electric (TE) or transverse magnetic slab modes, parallel-plate modes, that could be excited between the electrodes on the top surface and the grounded base, and a microstrip mode between the top electrodes and the grounded base. When coupling to extraneous modes occurs, there is a loss of power for the dominant CPW mode. Such a power loss degrades the optical modulator device""s modulation performance and the clarity of the output modulated optical signal is degraded. The amount of power lost to spurious or other extraneous modes depends on the field overlap between the dominant CPW mode and the other extraneous modes supported by the device.
The intended electrical guided mode for an optical modulator contains the frequency of an input or frequencies of input on the optical modulator for operating the optical modulator device. Typically, an optical modulator has a range of sets of frequencies that can be used as electrical inputs to modulate an optical signal. For proper operation of the optical modulator device, the intended electrical guided mode of the device must be such that the electric fields originating on the signal electrode must properly terminate on the adjacent ground electrodes without straying elsewhere in the modulator chip or package. The intended electrical guided mode of the optical modulator device will hereinafter be referred to as the dominant CPW mode of the optical modulator device.
Once the electric fields of the signal electrode penetrate through the buffer layer into the optical modulator chip, several other effects could occur. Depending on frequency, a CPW mode may couple with other extraneous electrical modes that the structure of the optical modulator device can support. These modes could either be highly dispersive slab modes, or could be zero-cut-off modes. Examples of extraneous modes are: transverse-electric (TE) or transverse magnetic slab modes, parallel-plate modes (that could be excited between the electrodes on the top surface and the grounded base), and microstrip mode (between the top electrodes and the grounded base). When coupling to extraneous modes occurs, there is a loss of power for the dominant CPW mode. Such a power loss degrades the optical modulator device""s modulation performance and the clarity of the output modulated optical signal is degraded. The amount of power lost to spurious or other extraneous modes depends on the field overlap between the dominant CPW mode and the other extraneous modes supported by the device.
Maintaining bias stability between the ground and the signal by preventing a charge build-up on the optical modulator chip is important for satisfactory operation of the optical modulator device. Otherwise, the optical modulation will not accurately represent the electrical signal that was inputted into the optical modulator as a result of bias instability caused by charge build-up on the optical modulator chip.
Accordingly, the present invention is directed to an optical modulator that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
The present invention provides an optical modulator with a structure to enhance bias stability so as to minimize signal loss.
The present invention provides an optical modulator with a structure and to prevent the introduction of extraneous modes into the modulated optical signal.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, an optical device of the present invention includes an optical modulator chip having a top surface, a back surface and side surfaces; at least one ground electrode and a signal electrode located over the top surface of the optical modulator chip; and a resistive layer applied on the back surface of the optical modulator chip.
In another aspect, the optical device of the present invention includes a grounded base having a top surface; an optical modulator chip having a top surface, a back surface and side surfaces, wherein the optical modulator chip positioned over the grounded base with the back surface of the optical modulator chip facing the top surface of the grounded base; and wherein a resistive layer is positioned between the grounded base and the optical modulator chip.
In another aspect, the optical device of the present invention includes a grounded base having a top surface; an optical modulator chip having a top surface, a back surface and side surfaces, wherein the optical modulator chip is positioned over the grounded base with the back surface of the optical modulator chip facing the top surface of the grounded base; at least one ground electrode and a signal electrode located over the top surface of the optical modulator chip; and a resistive layer positioned between the top surface of the grounded base and the back surface of the optical modulator chip.